Removable Polyurethane Encapsulants - American Chemical Society

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Downloaded by EMORY UNIV on August 24, 2015 | http://pubs.acs.org Publication Date: March 15, 1984 | doi: 10.1021/bk-1984-0242.ch024

Removable Polyurethane Encapsulants Κ. Β. WISCHMANN Sandia National Laboratories, 7472, Albuquerque, NM 87185

Castable thermoplastic polyurethane formulations with superior upper temperature capability have been developed for use as removable electronics packaging materials. Several formulations have been developed employing a two-step prepolymer processing technique. For example, a 58/42 (pbw) ratio of polyether to polyester diols reacted with bitolylene diisocyanate forms a prepolymer; this step is followed by reaction with a chain extender, 1, 4 butanediol to form the final polymer. This formulation resulted in a semi-crystalline material with a tensile strength of 8.2 MPa and a 275% elongation. Changes in the polyether/polyester ratio yielded materials of differing mechanical properties which have been attributed to differences in the microphase structure between hard and soft segments in these linear polyurethanes. These formulations were essentially free of creep over an 80 hour period at 70°C, the temperature of interest. These casting resins can be processed with a filler such as alumina or glass microballoons to reduce the coefficient of thermal expansion or change density. The formulations are soluble in polar organic solvents such as dimethylformamide. Functional electronic components have been successfully potted and depotted using these solvent removable formulations. 0097-6156/ 84/ 0242-0305506.00/ 0 © 1984 American Chemical Society In Polymers in Electronics; Davidson, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 1984.


306

POLYMERS IN ELECTRONICS

The purpose of this investigation was to improve the upper use temperature of solvent-removable encapsulants as compared with previously reported systems ( 1. )· These kinds of materials have been developed to provide a component re-work capability, that i s , they allow one to repair a sophisticated electronic package without discarding the entire, often costly assembly. Previous work has demonstrated the f e a s i b i l i t y of the removable encapsulant concept ^1,2) . In order for these encapsulants to be easily removed with solvent, they must either be linear or very lightly crosslinked. Unfortunately, polymers of this nature are generally susceptible to creep, especially at elevated temperatures. Past efforts produced linear TDI-polyether polyurethane encapsulants that were creep resistant at« ambient temperature but prone to creep at >40 C (about Tg). To improve these materials use temperature range (-40 C to 100 C), new polymers were developed which possess differing chemical structure and morphology. Polyurethanes were selected for investigation because of the wide range of physical properties that can be achieved. The thermoplastic polyurethanes studied here are segmented copolymers composed of alternating flexible polyether or polyester units (soft segments) which are connected to short rigid urethane groups (hard segments) ^3,4^. By varying the ratio of hard to soft segments, the chemical structure, and the molecular weight, dramatic changes in mechanical properties can be realized. These variables are directly related to the polymer's morphology resulting from phase separation of the hard and soft segments ( 5 ) · The hard phase regions in these materials act as virtual crosslinks which impart elastomeric-like properties to the system. Unlike thermosetting polymers, however, these materials can be dissolved in solvent or molded upon heating. Efforts were made to enhance the domain structure by increasing the amount of hydrogen bonding between hard and soft segments and altering the diisocyanate structure. It was f e l t that these changes would restrict the mobility of the polymer resulting in better creep resistance, increased strength and higher upper use temperature. In this paper optimized polymer formulations, processing techniques and properties are discussed. Experimental The materials employed in these formulations consisted of bitolylene diisocyanate (3,3 -dimethyl-4,4 -biphenylene diisocyanate, 136T Upjohn Co.) referred to as TODI, a polyether diol (tetramethyleneoxide polyol, Polymeg 2000, Quaker Oats Co.), a polyester diol (caprolactone polyol, PCP-200, Union Carbide Corp.) and a chain extender, 1,4-butanediol (Aldrich Chemical Co.). The formulations were prepared by a two-step procedure. In 1

1

In Polymers in Electronics; Davidson, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 1984.


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Removable Polyurethane

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the f i r s t step the appropriate ratio of polyether to polyester was reacted with the bitolylene diisocyanate (NCO/OH = 2) to form a prepolymer. In the second step stoichiometric amounts of 1,4butanediol were reacted with the prepolymer. The formulations were designated 100ET20, 64ET33 and 58ET34, where the f i r s t number refers to the weight ratio of polyether (ET) in the soft segment and the remainder is assumed to be the polyester, e.g., 58 parts by weight polyether, 42 parts by weight polyester, while the second number designates the weight percent of the diisocyanate based on the overall weight of the elastomer. This nomenclature has been employed elsewhere (6,7j. The formulation termed 100ET20 contained only the polyether and was used primarily for comparison purposes. See Tables 1 and 2 for polymer designations and formulations. Polymers of this nature can be polymerized either in solution or in bulk; in the latter case they are normally reacted at high temperatures, e.g., 100-150 C. Since our goal was a casting resin, the formulations were reacted in bulk and at lower temperatures to protect heat sensitive electronic components; furthermore, low reaction temperatures minimize side reactions that can lead to crosslinking and polymer insolubility. In this process the polyols and diisocyanates were mixed and allowed to react for about 25 minutes at 71 C to form the prepolymer formation while longer times resulted in material too viscous to cast or deaerate. After the indicated time, 1,4-butanediol was added followed by deaeration and subsequent encapsulation of a preheated (71 C) electronic device. A second deaeration of the encapsulated part is usually necessary. Pot l i f e for such a system is about 15 minutes. Final reaction or "cure" was 24 hours at 71 C. Mechanical testing was accomplished with an Instron Model 1130 using an ASTM tensile specimen D1708, after the specimens were aged for two weeks. Glass transitions, Tg, were obtained from a Perkin-Elmer Thermomechanical Analyzer TMS-1 in the penetrometer mode. Volume r e s i s t i v i t y measurements were made according to ASTM D-150 method. Dynamic mechanical properties were obtained from -140°C to +140°C at 11 Hz on a Rheovibron DDVII direct reading dynamic viscoelastometer. Rheovibron compliance corrections were made following a method suggested by Massa { 8 J . Results and Discussion The synthesis of these linear polymers follows a typical two step urethane addition sequence shown as follows: Step 1 HO-VWOH + 2 0CNRNC0 Macroglycol

Diisocyanate

^

OCNRNHCOO-AArOOCHNRNCO Prepolymer


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Step 2 ο

OCNR'NCO + HOR'OH


Prepolymer

1,4 Butanediol

ο

CNHR'NHCOR'OPolyurethane

These reactions are essentially chain extending reactions which yield the thermoplastic product. Difunctional materials were used throughout in an effort to maintain polymer linearity. Since a casting resin was desired, a l l reactions were of necessity carried out neat. A l l materials were used in the "as received" from, consequently, competing reaction such as the reaction of moisture with the diisocyanate may not have been avoided. An attempt was made to control allophanate formation (crosslinking reaction) by maintaining a mix and cure temperature no higher than 71 C j 3 ) . Completeness of reaction in the final product was determined by monitoring the disappearance of isocyanate via infrared analysis. Based on previous efforts to formulate these kinds of materials ·'.!')> attention was directed towards increasing r i g i d i t y in the hard and soft segment domain structure. This was accomplished by employing bitolylene diisocyanate as part of the hard segment and augmenting the polyether diol soft segment with a polyester d i o l . The polyester diol adds carbonyl groups to the chain backbone which should promote stronger hydrogen bonding than the polyether diol. The net effect from these modifications was increased mechanical strength, negligible creep at one of the higher use temperatures (71 C) when compared to a polymer system that did not contain any polyester d i o l . These modifications also introduced c r y s t a l l i n i t y and a resultant melting transition (Tm) which translates into improved thermomechanical properties. Crystallinity was established by x-ray measurements ( 9). Mechanical properties of three formulations designated 100ET20, 64ET33 and 58ET34 are shown in Table 3; the f i r s t system does not contain any polyester d i o l . As polyester diol content was increased tensile strength and modulus increased accordingly. The Tg also increased as well as the crystalline melting point, Tm. Dynamic mechanical properties, reflecting the same trends are shown in Figure 1. It is interesting to note that a wide use temperature range was achieved; that i s , l i t t l e change in modulus between 0° and 125°C. Since these polymers are thermoplastic, creep becomes an important property consideration in a casting resin. Creeg compliance measurements were made on 64ET33 at ambient, 40 C and 70 C employing a load approximately equal to 107 of the material's tensile strength. As shown in Figure 2, some i n i t i a l o


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Table 1. Component designation Component

Molecular Weight

Poly(tetramethylene oxide)

ET

530

ES

Poly(« -caprolactone) Bitolylene diisocyanate

Code

2000 264

TODI

90

BD

1,4 - butanediol


Table 2 . Elastomer formulations Molar Composition TODI/BD/ET/ES

Sample Designation

Weight % Hard Segment

100ET20

1/0.5/0.5/0

24

64ET33

1/0.5/0.16/0.34

38

58ET34

1/0.5/0.14/0.36

40

Table 3 . Thermoplastic polyurethane physical properties 100ΕΓ20

7.0 gm 4.0 gm 5.8 gm 1.0 gm Unfilled

6.55 (950 psi)

7.58(1100 psi)

Percent Elongation

600

275

35(5000 psi)

Shore D

T

m

U

220X 10 / ° C

(softening)

Volume Resistivity, ohm-cm

Unfilled

A / 0 Filled 2

3

10.3(1500 psi)

13.1(1900 psi)

20

124 (18,000 psi)

500

1275 (185,000 psi)

145 (21,000 psi) 51

45

24

Coeff. Expansion (-52 to 72 °C)

7.0 gm Polymeg 2000 5.0 gm PCP-200 6.8gm TODI 1.1 gm 1,4-Butanediol

Polymeg 2000 PCP-200 TODI 1,4-Butanediol

"Tensile Strength^ MPa

Modulus, MPa

58ET34

64ΕΓ33

7.8 gm Pol y meg 2000 2,lgmT0DI 0.33 gm 1,4-Butanediol

6

160 X 10" /°C

6

100X 70" /°C

6

150 X 10" /°C

-59°C

-53°C

-33°C

120°C

147°C

168°C 13

12

1.3X 10 (500v)

12

1.04 X 10 ( 1000 v)

3.27 X 10 (500v) 3.01 X 10 ( 1000 v)

13

Tensile testing conducted at ambient temperature and a crosshead speed of 0.2" min 1. _1


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58ET34 64ET33

-150

-50 50 Temperature, C

150

e

Figure 1. Dynamic mechanical p r o p e r t i e s o f l i n e a r polyurethane encapsulants at 11 Hz. Formulation nomenclature: the f i r s t number r e f e r s to the weight r a t i o of polyether (ET) i n the s o f t segment and the remainder i s assumed to be the p o l y e s t e r ; the second number designates the o v e r a l l weight percent of the d i i s o c y a n a t e .

30 -

ο

Time, h Figure 2. Creep compliance v s . time f o r l i n e a r polyurethane encap sulants at constant l o a d . See F i g u r e 1 f o r nomenclature. Key (64ET33): O , 70 ° C ; • , 40 ° C ; and ^ , ambient. Key (58ET34): O , 70 ° C .


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c r e e p o c c u r s i n t h e f i r s t 5-6 h o u r s and t h e n e s s e n t i a l l y c e a s e s o v e r t h e n e x t 70 h o u r s o b s e r v a t i o n t i m e . S i n c e 64ET33 showed e v i d e n c e o f c r e e p a t 70 C., more p o l y e s t e r d i o l was added t o t h e f o r m u l a t i o n i n an e f f o r t t o r e d u c e c r e e p . As a r e s u l t 58ET34 e x h i b i t e d n e g l i g i b l e c r e e p (

Removable Polyurethane Encapsulants - American Chemical Society

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